In a wireless communication system, a transmitting end usually uses multiple antennae to acquire a higher transmission rate. The multiple antennae can lead to signal-to-noise ratio improvement and support more spatial multiplexing layers. Compared with an open-loop Multi-Input Multi-Output (MIMO) technology where the transmitting end does not use Channel State Information, CSI, a closed-loop MIMO precoding technology using the CSI information will be higher in capacity, and is a transmission technology widely used by current mainstream 4G standards. The core idea of the closed-loop MIMO precoding technology refers to that a receiving end feeds channel information back to the transmitting end, and the transmitting end uses some transmitting precoding technologies according to the obtained channel information, thereby greatly improving the transmission performance. For single-user MIMO, a precoding vector relatively matched with channel feature vector information may be directly used for transmitting precoding. For multi-user MIMO, more accurate channel information is also needed to perform interference elimination. Therefore, acquisition of the channel information of the transmitting end plays an important role.
In some 4G technologies such as a Long Term Evolution/Long Term Evolution-Advanced, LTE/LTE-A, 802.16m standard specification, downlink channel information of a Frequency Division Duplexing, FDD, system is acquired by using a general flow as follows.
In step S1, a transmitting end (base station) transmits downlink Channel State Information-Reference Signals (CSI-RSs) to a receiving end. Generally speaking, each antenna transmits a portion of CSI-RSs. CSI-RSs transmitted by different antennae are staggered in positions of a time frequency domain or code domain, which can keep the orthogonality from mutual interference. Each antenna corresponds to a CSI-RS port, and a channel measurement reference signal is used for measuring channel information. In LTE-A, transmission of CSI-RSs via a maximum 8-antenna port of a base station side is supported. The base station also transmits related position information and transmitting period information of a Radio Resource Control, RRC, signaling configuration CSI-RS to a terminal. Transmitting contents of a reference signal at the base station side are determined according to some pre-appointed rules, and the terminal can accurately learn of transmitting content of the reference signal at each port of the base station side at each time frequency position.
In step S2, the terminal receives configuration information of the CSI-RS transmitted by the base station side, and the terminal receives and detects CSI-RS at transmitting time frequency resource positions of multiple reference signal ports informed by signaling. The received CSI-RS is obtained on each receiving antenna of the terminal side. Since the terminal and the base station appoint contents of the reference signal transmitting signals at multiple time frequency resource positions of multiple transmitting ports, the terminal can accurately learn of downlink reference signal transmitting signals. Therefore, the terminal may perform downlink channel estimation according to the received reference signal to obtain downlink channel response information between a terminal-side receiving antenna port and a base station-side transmitting antenna port. During downlink channel estimation, it is necessary to consider influences of noise and interference mixed into the actual signal during receiving of the reference signal, a Least Square, LS, algorithm, a Minimum Mean Square Error, MMSE algorithm, an Interference Rejection Combining, IRC, algorithm and other algorithms may be adopted to perform channel estimation, and a downlink channel matrix matched with the number of transmitting ports at multiple time frequency resource positions is obtained finally.
In step S3, the terminal may estimate a channel response between a receiving antenna and multiple transmitting antenna ports according to contents of the transmitting signals for reference signals at multiple reference signal ports and receiving reference signals on multiple receiving antennae. That is, channel matrix corresponding to multiple time frequency resource positions may be obtained, and optimal CSI information may be further calculated according to the channel matrices. The CSI generally includes three types of information, namely a Precoding Matrix Indicator (PMI), Channel Quality Indicator (CQI), and Rank Indicator (RI). The three types of information feed back and recommend a precoding matrix, channel quality information and number of transmission layers to the base station respectively. The terminal feeds the calculated CQI/PMFRI information back to the base station via a control channel of an uplink physical layer or a data channel of the uplink physical layer. The base station determines the number of transmission layers, a coding modulation mode and transmitting precoding on the basis of the feedback information of the terminal.
It can be observed that the downlink CSI-RS plays a very important role in a CSI acquisition process, and generally influences the accuracy of the precoding information, the channel quality information and information about the number of transmission layers, and further significantly influences the transmission performance of MIMO.
Downlink CSI-RS s adopted in the 4G standard are all periodic CSI-RSs. In the time domain, in view of that a channel is not suddenly changed, the change has a certain time domain relevancy and the related time length is longer than a subframe duration 1 ms, therefore, it is unnecessary to transmit all subframes. All User Equipments UEs may share the CSI-RS, therefore the CSI-RS is transmitted periodically in general. The concept of a periodic reference signal is that the base station transmits the CSI-RS s at a certain periodic interval, and a transmitting position may have different subframe position offsets. For example, a CSI-RS period and a subframe offset in LTE-A are defined as follows.
A specification in an LTE standard 36.211 is as shown in Table 1, i.e., CSI-RS subframe configuration.
TABLE 1CSI-RS subframe configurationCSI-RS-SubframeConfigCSI-R SperiodicityCSI-RS subframe offsetICSI-RSTCSI-RS (subframes)ΔCSI-RS (subframes)0-45ICSI-RS 5-1410ICSI-RS − 5 15-3420ICSI-RS − 1535-7440ICSI-RS − 35 75-15480ICSI-RS − 75
In Table 1, ICSI-RS is a configuration parameter of the CSI-RS, of which the value ranges from 0 to 154. Different values correspond to different CSI-RS periods and subframe offsets. FIG. 1 shows a transmitting diagram of subframe positions corresponding to some CSI-RS configuration examples, i.e., corresponding respectively to configurations of ICSI-RS=0, ICSI-RS=2 and ICSI-RS=5.
At a frequency domain position, a CSI-RS exists inside each Physical Resource Block pair, and transmitting patterns of the same port inside different PRB pairs are the same. A pattern of the CSI-RS is as shown in FIG. 2. The PRB pairs may refer to the specification in the LTE standard 36.211, typically including 12 frequency domain subcarriers and 14 time domain Orthogonal Frequency Division Multiplexing (OFDM) symbols.
In an LTE system, it is specified that 40 Resource Elements, REs, in one PRB pair may be used as CSI-RSs and are divided into 5 patterns, and each pattern includes 8 REs, as shown in the above figure. Each port of the CSI-RS reference signal occupies an RE in one PRB pair averagely, and all ports belonging to one CSI-RS resource are required to be limited within a pattern #i as shown in FIG. 2. At present, a CSI-RS supports 8 ports maximally. Therefore, when the number of ports is 8, there are 5 candidate positions. When the number of ports is 4, there are 10 configurable positions. When the number of ports is 2, there are 20 configurations.
When a base station in an LTE-A system transmits a CSI-RS, precoding process cannot be performed generally in the existing art. The main reason is that multiple UEs in a cell share the CSI-RS, and if precoding is to be performed on the CSI-RS, precoding can be performed only based on features of a channel between the base station and a UE, which may influence measurements of other UEs, that is, other UEs cannot accurately measure a physical channel between Nr receiving antennae and Nt transmitting antennae, and precoding performed based on features of other UE channels will make it unable to accurately calculate and report own CSI. Certainly, in a large-scale antenna communication system discussed currently, when there are quite a few antennae, to save the reference signal overhead and reduce the feedback complexity to the greatest extent, in some scenarios where a multi-path scattering ratio is relatively low, the base station may transmit a periodic precoding CSI-RS, and a precoded CSI-RS is called as a beam measurement reference signal generally. FIG. 3 shows a transmitting policy for a periodic beam reference signal, and energy of each beam reference signal is centralized in a certain direction to form a directional beam, and a beam measurement reference signal is transmitted at a time period interval. A polling is performed among a group of beam reference signals.
In addition to the periodic CSI-RS reference signal described above, a non-periodic CSI-RS reference signal is recently proposed. The non-periodic CSI-RS is an instant trigger reference signal, which is dynamically transmitted for channel measurement of a specific UE or UE group in general, and cannot be continuously transmitted and only exists in a subframe. Therefore, non-periodic reference signal trigger information is carried in a Physical Downlink Control Channel (PDCCH) or an Enhanced-PDCCH (ePDCCH).
After learning of a transmitting position of the non-periodic CSI-RS, the terminal may perform the reference signal detection at the corresponding position. Like the periodic CSI-RS, the transmitting content of the non-periodic CSI-RS may be pre-acquired by the terminal. Therefore, a downlink channel response between a terminal receiving antenna and a base station transmitting antenna may be estimated, thereby acquiring a channel matrix.
There are two typical non-periodic reference signal transmitting modes. One transmitting mode is transmission in a Physical Downlink Shared Channel (PDSCH) of a user needing to use a non-periodic CSI-RS for measurement. The other transmitting mode is that a non-periodic CSI-RS contention resource pool is allocated to all users in a cell and then resources are configured to different users based on the basis of the resource pool. As shown in FIG. 4, the non-periodic CSI-RS contention resource pool may be a set of transmitting resource positions of periodic CSI-RS.
It is observed that the non-periodic CSI-RS is generally oriented to a specific user or a specific user group instead of all users in the cell. Therefore, the non-periodic CSI-RS may support a precoding manner, and can effectively decrease the number of ports, and may further reduce the calculation amount of CSI feedback. Thus, the non-periodic CSI-RS may select to be transmitted in a precoding beam reference signal form or a non-precoding non-beam reference signal form as required.
Some basic knowledge about measurement reference signal transmission in the existing art was introduced above, including precoding reference signals for periodic reference signals and non-periodic reference signals and non-precoding reference signals. In the existing art, all parameters related to reference signals are determined and configured to the terminal by the base station, or the terminal and the base station make some appointments to adopt relatively fixed parameters.
As for a Time Division Duplexing (TDD) system, downlink channel information is mainly acquired in a reciprocity manner. A general flow for acquiring the downlink channel information of the TDD system is as follows.
In step B1, the base station configures Sounding Reference Signals (SRSs) for channel information measurement, and the base station may inform the terminal of related SRS transmitting information such as an SRS transmitting position, a transmitting period and a frequency domain occupation bandwidth via a PDCCH signaling or a high-layer RRC signaling.
In step B2, the terminal receives a configuration signaling transmitted by the base station, and transmits an SRS reference signal on a resource indicated by the base station according to a method indicated by the base station. The base station and the terminal appoint the transmitting signal content of SRSs at multiple time frequency resource positions.
In step B3, the base station receives SRS reference signal at SRS transmitting positions of multiple UEs. Since the base station can accurately learn of SRS transmitting signals, the base station may perform channel estimation according to the received reference signals to obtain uplink channel response information between a base station-side receiving antenna port and a terminal-side transmitting antenna port. During channel estimation, it is necessary to consider actual influences of noise and interference doped during receiving of the reference signal, an LS algorithm, an MMSE algorithm, an IRC algorithm and other algorithms may be adopted to perform estimation, and an uplink channel matrix matched with the number of transmitting ports at multiple time frequency resource positions is obtained finally.
In step B4, the base station performs channel reciprocity according to the obtained uplink channel matrix between the base station-side receiving antenna and the terminal-side transmitting antenna, to obtain a downlink channel matrix between a base station-side transmitting antenna and a terminal-side receiving antenna. Information such as precoding, channel quality, the number of transmission layers and so on, may be judged according to the downlink channel matrix.
It can be observed that the SRS plays a very important role in an uplink/downlink state information acquisition process, and generally influences the accuracy of the precoding information, the channel quality information and information about the number of transmission layers, and further significantly influences the transmission performance of MIMO.
The SRS is a reference signal transmitted by each terminal and used for measuring related channel information of the present terminal, and is a measurement reference signal of a specific characteristic of a UE rather than a reference signal shared by multiple users. Therefore, the SRS may be a precoding beam reference signal or may be a non-precoding non-beam measurement reference signal, and may be determined according to measurement demands, channel features and the like specifically.
In the LTE-A standard, the SRS is mainly designed on the basis of a multiple access mode of Single-Carrier Frequency-Division Multiple Access (SC-FDMA) at present. Under the limitation of a single-carrier characteristic of LTE-A uplink SC-FDMA, an uplink peak-to-average ratio and the like, the design of the SRS is greatly different from that of the downlink CSI-RS. A future uplink system may adopt an Orthogonal Frequency Division Multiple Access (OFDMA) mode, and therefore the design of the SRS may be considered to be similar to that of a downlink measurement reference signal.
The problem in the existing art is that: regardless of transmission of the CSI-RS or the SRS, the terminal does not participate in a reference signal parameter determination process, and the parameter is completely determined by the base station. As for the periodic reference signal, in a general traditional scenario, the method may not be extremely obvious in performance loss. But, in a scenario of the presence of some special users or special demands, including a terminal having severe delay demands, robustness demands and CSI quantization precision demands, or in a scenario where the interference is very large and there are many users, since the total perception of the base station for measurement reference signal demands is poorer than that of the terminal, no participation of the terminal in determination of the reference signal parameter will bring the system performance loss. Particularly for the non-periodic reference signal, the terminal has a more urgently demand to participate in determination of the reference signal parameter.